Technical Abstract:
Soil Loss Tolerance (T) values have been used by the Natural Resources Conservation Service in conservation planning. T values are used as a basis for soil conservation planning and regulations in this country, and adaptations of the U.S. standard have been adopted around the world. The T value was defined by scientists as the amount of soil that could be lost without a decline in fertility, thereby maintaining crop productivity indefinitely. The values set for U.S. soils are inadequate for 2 reasons: they are based on outdated and poor scientific evidence; the economic, environmental, and social issues have changed. We propose a new concept of soil loss tolerance, targeting on-site impacts; and targeting off-site impacts. We suggest that on-site erosion rates be tied to the balance of organic content of the soil. Increases/maintenance of high levels of organic material in the soil should be indicative of soil building and sustainable agricultural conditions. Soil organic content is a measurable and understandable criterion. We suggest that off-site impact of erosion be be tied to sediment balances in stream and river systems. The Mississippi River is by far the largest carrier of sediment in the continental U.S. The sediment load in the Mississippi has decreased dramatically, at the same time erosion rates in the river basin have increased due to development of agriculture. Sediment is being stored within the river basin in large quantities, which has important implications for flooding and navigation problems. We suggest that off-site erosional impacts be dependent upon creating a different andmore balanced rate of sediment buildup within stream and river systems in the U.S. This would effect a reduction in the amount of sediment stored until a more natural balance is reached.